The invention relates to a fan lade fragment containment assembly for a gas turbine engine with a compressible metal foam layer between an inner deformable layer and an outer containment layer.
The fan of a gas turbine engine is mounted on a rotatable hub with an array of fan blades projecting radially from the hub with a fan casing encircling the blades. It is possible for a fan blade to separate from the hub or a portion of the fan blade forming a fragment which is potentially hazardous if not contained within the engine.
Blades may become detached through metal fatigue or more commonly through a foreign object ingestion into the engine such as birds or hail stones. Foreign objects and any broken-blade fragments are propelled tangentially and axially rearwardly. Foreign objects are usually discharged through the bypass duct of the engine, however heavy blade fragments must be retained within the fan casing structure itself using a softwall containment; design to embed the fragments.
For small diameter engines a hard walled metallic case may be sufficient to deflect blade fragments axially rearwardly. In larger diameter engines the blade fragments are preferably retained within compressible honeycomb structures on the interior of the case and are often contained with ballistic fabric that encircles the fan casing.
The demands of blade fragment containment are balanced by the demands for low weight and high strength. Earlier systems are now being replaced with designs and incorporate lightweight high strength ballistic fabrics such as Kevlar(trademark) wrapped in multiple layers around a relatively thin deformable metal support case. In effect, the fan case metal structure serves as a deformable support to maintain the proper shape and the fabric wrapping serves as a ballistic containment layer that deflects radially outwardly capturing blade fragments. The metal case is punctured locally but retains its shape and structural integrity of the fan case after impact. The punctured metal case continues to support the ballistic containment fabric and maintains the clearance between the blade tips and fan case.
U.S. Pat. No. 6,059,524 to Costa et al. describes a penetration resistance fan casing wherein the ballistic fabric wrapped around the metal fan case during impact is released and permitted to deform by becoming detached from a forward edge of the metal fan case. An important feature of fabric containment systems is that the containment fabric""s penetration resistance is enhanced by slackening of the fabric and distribution of the tensile forces throughout the structure that encircles the fan case. The ballistic fabric acts as a tensile medium for transferring the radial impact force from the local area of the blade fragment to the metal fan case. A slackening or releasing of the tension within the ballistic fabric enables distribution of the impact forces and improves the penetration resistance of the ballistic fabric as kinetic energy of the blade fragment is reduced.
It is an object of the present invention to provide an improved blade containment system that minimizes damage to the adjacent nacelle.
It is a further object of the invention to minimize the diameter of the nacelle required to accommodate the containment system.
It is a further object of the invention to reduce or impede fragmentation of the released blade during impact with the containment system.
It is a further object of the invention to minimize the weight of the blade containment system as a function of the kinetic energy absorbed.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.
The invention provides a fan blade fragment containment assembly for a gas turbine engine having a fan case surrounding the fan and having a circumferential axially extending blade fragment impact zone. A relatively slack, low tension deformable layer of ballistic fabric such as Kevlar(trademark) or Zylon(trademark) for example is mounted to the fan case outer surface and envelopes the impact zone. A compressible (ex. metal foam or honeycomb) layer is mounted on an outer surface of the deformable layer. A friction reduction layer, of Teflon(trademark) or composite graphite based fabric, is mounted on an outer surface of the compressible layer and a relatively-taut, higher tension containment layer of Kevlar(trademark) or Zylon(trademark) fabric is mounted on an outer surface of the friction reduction layer.
On impact the blade fragment pierces through the fan case, deformable-layer and compressible layer and is captured by the containment layer. The blade fragment loses kinetic energy and sharp edges are somewhat blunted as a result of passing through the metal fan case, deformable fabric layer and compressible metal foam layer.
The containment layer extends radially on impact and under tension the containments layer transfers the impact forces circumferentially crushing the compressible layer. The slack created by the crushing action reduces tensile stresses and improves the capacity of the containment layer to resist penetration. The friction reduction layer enhances the distribution of impact forces about the circumference of the compressible layer by reducing local shear stresses between the compressible layer and the containment layer.
The use of light weight metal foam and light weight Kevlar(trademark) ballistic fabrics results in a minimization of the containment system weight: relative to the kinetic energy it is capable of absorbing. Permitting the blade fragment to penetrate to the fan case, a first inner deformable Kevlar(trademark) or Zylon(trademark) layer and the metal foam while being contained by the outer containment Kevlar(trademark) or Zylon(trademark) layer reduces the kinetic energy while preventing fragmentation of the released blade which would occur for example if a hard or less deformable containment system were utilized. Compression of the metal foam layer, results in a reduced deflection of the containment layer during containment of the blade fragment compared with prior art systems. The prior art relies on a greater degree of deflection of the outer containment layer. In contrast, the present invention includes compression or crushing of the metal foam compressible layer, which enables reduced deflection of the outer containment layer of ballistic fabric.
Further provision of a low friction, friction reducing or lubricating layer between the outer containment layer and the compressible or crushable layer of metal foam enhances the distribution of impact forces from the local area of blade contact to the entire fan case. Reduction of friction enables rapid deflection of the outer containment layer and crushing of the relatively large zone of the compressible layer thereby reducing local sheer stresses and transferring tension produced on impact, enhancing the crushing of the metal foam, redistribution of impact forces and avoids local stresses by distributing stresses more efficiently throughout the encircling fan case.